Home Page

Branton Campbell in the lab

Branton J. Campbell

Professor

Department of Physics & Astronomy

Brigham Young University

Provo, UT 84602, USA

Tel: 801-422-5758, Fax: 801-422-0553

Email: branton_campbell[at]byu.edu

//physics.byu.edu/faculty/campbell/

Research Interests

I apply state-of-the-art x-ray and neutron scattering techniques to study local and long-range structures in a variety of complex solids, including fast-ion conductors, ferroelectric relaxors, high-temperature superconductors, and colossal magnetoresistive manganites, where nanoscale structural features influence macroscopic physical properties. This includes the development of symmetry-mode analysis (through the tools of the ISOTROPY Software Suite) for the determination, refinement and interpretation of distorted structures involving lattice strains, atomic displacements, magnetic moments and occupational orderings at both commensurate and incommensurate wavevectors.

Gamma-Alumina Nanoparticles

Nanoparticles of gamma-alumina obtained from a novel synthetic solvent-deficient method show promise as improved industrial catalyst-supports. X-ray PDF analysis reveals that they are born with a high concentration of defects that locally resemble boehmite. As the nanoparticles are annealed to successively higher temperatures, the boehmite-like defects heal gradually rather than disappearing in an abrupt phase transition, which explains several previously misunderstood of properties of the gamma phase.

Selected Publications

Rietveld refinement of structural distortion-mode amplitudes

B. J. Campbell and H. T. Stokes (et al.)

For any crystal structure that can be viewed as a low-symmetry distortion of some higher-symmetry parent structure, one can represent the details of the distorted structure with a list of distortion-mode amplitudes rather than the traditional list of xyz atomic coordinates. After importing mode definitions from the ISODISPLACE software, TOPAS Academic can now directly refine structural distortion-mode amplitudes. In this article, we demonstrate that the distortion-mode basis is well-suited to a Rietveld refinement of the room-temperature structure of WO3, which has 24 displacive atomic-coordinate degrees of freedom. Only five distortion-mode amplitudes are needed to capture the essential features of this structure.

Order parameters for phase transitions to structures with one-dimensional incommensurate modulations

Harold T. Stokes, Branton J. Campbell, and Dorian M. Hatch

Phase transitions that result in incommensurate structural modulations are widely observed in crystalline solids and are relevant to a broad range of physical phenomena in magnetic, electronic, optical and structural materials. While the (3+1)-dimensional superspace-group symmetries associated with one-dimensional modulations have been tabulated, the order parameters that produce these modulations have not been explored in detail. Here, using group-theoretical methods, we present a unique and exhaustive enumeration of the isotropy subgroups (and their corresponding order-parameter directions) belonging to irreducible representations of the (3+1)-dimensional superspace extensions of the 230 crystallographic space groups at all incommensurate k points. The vast majority of experimentally observed incommensurately modulated structures have order parameters belonging to one of these subgroups.

Microscopic annealing process and its impact on superconductivity in T '-structure electron-doped copper oxides

Branton J. Campbell (et al.)

High-transition-temperature superconductivity arises in copper oxides when holes or electrons are doped into the CuO2 planes of their insulating parent compounds. Whereas hole doping quickly induces metallic behaviour and superconductivity in many cuprates, electron doping alone is insuffcient in materials such as R2CuO4 (R is Nd, Pr, La, Ce and so on), where it is necessary to anneal an as grown sample in a low-oxygen environment to remove a tiny amount of oxygen in order to induce superconductivity. Here we show that the microscopic process of oxygen reduction repairs Cu deficiencies in the as-grown materials and creates oxygen vacancies in the stoichiometric CuO2 planes, effectively reducing disorder and providing itinerant carriers for superconductivity. The resolution of this long-standing materials issue suggests that the fundamental mechanism for superconductivity is the same for electron- and hole-doped copper oxides.

ISODISPLACE: a web-based tool for exploring structural distortions

Branton J. Campbell, Harold T. Stokes, David E. Tanner, and Dorian M. Hatch

ISODISPLACE is a new internet-server tool for exploring structural phase transitions. Given parent-phase structural information, it generates atomic displacement patterns induced by irreducible representations of the parent space-group symmetry and allows a user to visualize and manipulate the amplitude of each distortion mode interactively. ISODISPLACE is freely accessible at http://stokes.byu.edu/isodisplace.html via common internet browsers.

Diffuse scattering in the layered perovskites

B. J. Campbell

Single-crystal X-ray and neutron diffuse scattering techniques have been used to probe a variety of short-range displacive and magnetic orderings in the layered transition-metal perovskites which arise due to the intimate coupling of their structural, electronic and magnetic degrees of freedom. The present work reviews recent analyses that have successfully modeled short-range intralayer and/or interlayer correlations in these materials leading to useful physical insights, and describes the methods employed within a general formulation of the disordered structure function.